1. Field of the Invention
The present Invention relates to an external resonator type wavelength-variable semiconductor laser light source for optical coherent communication, which varies the oscillation wavelength of the semiconductor laser while continuing the phase of oscillated light.
2. Background Art
FIG. 2 is a block diagram showing a conventional external resonator type wavelength-variable semiconductor laser light source. In FIG. 2, reference numeral 1 indicates a semiconductor laser, reference numeral 1A indicates an antireflection film, reference numeral 3 indicates a diffraction grating, reference numeral 4 indicates a rotating stage, reference numerals 5 and 6 indicate lenses, reference numeral 7 indicates an arm, and reference numeral 10 indicates a fixed plate.
In the arrangement of FIG. 2, one end face of semiconductor laser 1 is coated with antireflection film 1A. From the end face with the antireflection film, outgoing beam 2B is outputted. The outgoing beam 2B is transformed into a collimated beam by lens 6 and is incident on diffraction grating 3 which has grooves whose interval is d, the diffraction grating being placed in a direction inclined by angle .theta..sub.0 to a direction along the optical axis of semiconductor laser 1.
At this time, the central part of the diffraction grating 3 and the other end face without an antireflection film of the semiconductor laser 1 form an external resonator which has initial length L.sub.0. Therefore, semiconductor laser 1 oscillates at initial wavelength .lambda..sub.Gr0 which corresponds to a mode selected among initial external resonance longitudinal modes .lambda..sub.FP0 (the interval of the modes is represented by .left brkt-top..lambda..sub.FP0.sup.2 /2L.sub.0 .right brkt-bot.) so as to satisfy a condition that the optical loss in the resonator is the smallest via the diffraction grating 3. Here, initial external resonance longitudinal modes .lambda..sub.FP0 satisfy the following formula (1), and initial wavelength .lambda..sub.Gr0 is represented by the following formula (2). Wavelength .lambda. of semiconductor laser 1 can be varied by rotating diffraction grating 3. EQU .lambda..sub.FP0 =2L.sub.0 /m (1)
wherein:
.lambda..sub.FP0 : initial external resonance longitudinal modes PA1 L.sub.0 : initial length of resonator PA1 m: order of each mode EQU .lambda..sub.Gr0 =2d sin .theta..sub.0 ( 2) PA1 .lambda..sub.Gr0 : initial oscillation wavelength PA1 d: interval of the grooves PA1 .theta..sub.0 : initial angle of inclination of diffraction grating PA1 .DELTA.L: variation of the length of resonator PA1 L.sub.A : distance from the top of arm 7 to the center of diffraction grating PA1 .DELTA..theta.: angle of rotation EQU .lambda..sub.Gr =2d sin (.theta..sub.0 +.DELTA..theta.) (4) PA1 .lambda..sub.Gr : oscillation wavelength
wherein:
In FIG. 2, diffraction grating 3, which acts as an external mirror of semiconductor laser 1, is fixed on rotating stage 4 which includes a mechanism which moves in parallel with the optical axis of the semiconductor laser. Furthermore, the rotating stage 4 is connected to fixed plate 10 via arm 7. Therefore, the rotational motion of diffraction grating 3 is transformed into parallel motion, and length L of the external resonator varies by .DELTA.L. Here, external resonance longitudinal modes .lambda..sub.FP (the interval of modes is represented by ".lambda..sub.FP.sup.2 /[2 {L.sub.0 +.DELTA.L}]") which correspond to the rotational angle .DELTA..theta. of diffraction grating 3, and wavelength .lambda..sub.Gr which is selected as the oscillation wavelength via diffraction grating 3 are respectively represented by the following formulas (3) and (4). ##EQU1## wherein: .lambda..sub.FP : external resonance longitudinal modes
wherein:
Wavelength error .DELTA..lambda., that is, the difference between oscillation wavelength .lambda..sub.Gr chosen by means of diffraction grating 3 and corresponding external resonance longitudinal mode .lambda..sub.FP, is represented by the following formula (5). Therefore, if length 1 of arm 7 is set such that the condition "L.sub.A =m.multidot.d" is satisfied, the wavelength error .DELTA..lambda. is always maintained at zero, regardless of the rotational angle .DELTA..lambda. of diffraction grating 3. ##EQU2## wherein: .DELTA..lambda.: wavelength error
Accordingly, diffraction grating 3, arm 7, and fixed plate 10 constitute a linear feed for wavelengths (hereinafter, the constitution is referred to as a "sine bar"). Therefore, it is possible to vary the oscillation wavelength under phase-continuous conditions, wherein order m of the oscillation mode is fixed (i.e., the jump of the oscillation mode does not occur).
On the other hand, outgoing beam 2A from the other end face without an antireflection film of semiconductor laser 1 is transformed into a collimated beam via lens 5; and the collimated beam becomes an output beam from the external resonator type wavelength-variable semiconductor laser light source.
In the external resonator type wavelength-variable semiconductor laser light source using the sine bar, the wavelength error .DELTA..lambda. is always maintained at zero for the angle .DELTA..theta. of rotation of the diffraction grating; thus, it is possible to vary the oscillation wavelength under phase-continuous conditions, wherein order m of the oscillation modes is fixed (i.e., the jump of the oscillation mode does not occur). However, if there occurs any setting error in the interval of grooves in the diffraction grating, the position of the fixed plate, the length of the arm, or the position at which the beam is incident on the diffraction grating, and the like, wavelength error .DELTA..lambda. gradually increases for the rotational angle .DELTA..theta.. Then, if wavelength error .DELTA..lambda. exceeds the interval of the external resonance longitudinal modes, the jump of oscillation mode occurs. In this case, the range of wavelength-variation under phase-continuous conditions is limited to a narrow one.
For example, if the position of the incident beam on the diffraction grating has an error in a direction perpendicular to the optical axis of the semiconductor laser, the position error of the incident beam is included in formula (5). In this case, even if length 1 of arm 7 is adjusted such that the condition "L.sub.A =m.multidot.d" is satisfied, the wavelength error .DELTA..lambda. gradually increases for the angle .DELTA..theta. of rotation of the diffraction grating. Therefore, for the purpose of achieving a wide range of wavelength variation under phase-continuous conditions, the position error of the incident beam on the diffraction grating must be minimized as much as possible.
However, in the sine bar arrangement of the conventional external resonator type wavelength-variable semiconductor laser light source shown in FIG. 2, the physical length of the external resonator is changed by moving the diffraction grating at the time of wavelength variation; thus, the conventional light source has a problem, that is, a problem occurs in that the accuracy of parallel movement of the rotating stage directly influences the position error of incident beam.